1. Field of the Invention
The present invention relates to a process for preparing water dispersible negative-type photosensitive compositions. Particularly, the water dispersible negative-type photosensitive compositions prepared by the process of the present invention have low solvent content and are useful as resists for forming printed circuits, printing plates or solder masks.
2. Description of the Related Arts
Photosensitive material used in the process of manufacturing printed circuit board may be classified into 3 categories: photoresist for etching, dielectric for build-up processes and solder resist. The photoresist for etching is used in the etching of copper-conductor circuits and should be stripped from the surface of copper-conductor circuits after etching. The dielectric for build-up processes is used for the insulation of layers of copper-conductor circuits. The solder resist is applied onto partial circuits after the manufacturing of copper-conductor circuits has been finished. In addition, the photosensitive material can be classified liquid type or dry film type according to their outward appearance. The liquid type photosensitive material is usually coated onto circuit boards by screen printing, roller coating, curtain coating or dip coating. Dry film type photosensitive material is first applied onto a transparent polyester substrate, then the photosensitive layer is heated and laminated to the circuit board substrate.
Further, photosensitive material can also be classified as solvent-developable or alkaline aqueous-developable according to the pattern of development. The former uses organic solvent as developing solution, which is a potential irritant and harmful to staff during operation. Further, contamination arising from evaporation increases the recovery costs. Thus, solvent-developable photosensitive material is gradually being replaced by alkaline aqueous-developable material. The developing solution used in the alkaline aqueous-developable material is mostly sodium carbonate aqueous solution. Because the binder is present in the photosensitive composition contains carboxyl groups, it can be neutralized by sodium carbonate to form salts during development. This causes the photosensitive composition which has not been exposed to be emulsified and dispersed in the water. Thus, the purpose of the development is achieved.
The components of the photosensitive composition include binder, photomonomer, photoinitiator, thermal inhibitor, storage stabilizer, plasticizer and other additives. Because the components are not completely compatible each other, the addition of 30-70% of organic solvents to dissolve and disperse each component is necessary. See, e.g. U.S. Pat. Nos. 4,418,138, 4,537,855, 4,604,343, 5,364,737, 5,393,643, 5,387,494, 5,389,495 and 5,411,837. The type of organic solvents depends on the purpose of the photosensitive composition, including dichloromethane, chloroform, tetrachloromethane, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, 2-butyl alcohol, isobutyl alcohol, n-pentyl alcohol, 2-methyl-1-butanol, isopentyl alcohol, n-hexyl alcohol, propyleneglycol monomethyl ether, dipropyleneglycol monomethyl ether, carbitol, acetone, methyl ethyl ketone and propyleneglycol monomethyl ether acetate, etc.
The bulk use of organic solvents is potentially risky for the operators and increases the recovery costs, thus decreasing the competitiveness of this material. The photosensitive compositions prepared by the methods of the prior art contain at least 20%-30% of organic solvents (See, e.g. Barr, U.S. Pat. No. 5,389,495 and Bttomley et al, U.S. Pat. No. 5,411,837). Thus, the present invention aims to replace organic solvents with water while maintaining the desired properties of a photosensitive composition for making electric circuit substrates.